1. Field of the Invention
This invention relates to lanthanum sulphur compounds which are stabilized by calcium, barium, or strontium and have desirable properties for use in thermoelectric energy conversion.
2. Background Discussion
Thermoelectric materials are substances which, when subjected to a temperature differential across the material, produce a voltage drop and a current flow through the material. Such materials are used in remote power generation, for example, in a spacecraft as a source of electrical power. One end of the material is exposed to heat, for example, from a radioisotope heat source, and the other end is at a substantially lower temperature caused by intentional heat removal such as radiative coating, subjecting the material to a thermal gradient which causes power to be generated by the material. Thus, heat energy is directly converted to electrical energy to provide power for the spacecraft.
In a material such as lanthanum sulfide, certain crystallographic phases may have desirable thermoelectric properties, while other phases may be electrical insulators. For example, the cubic phase form of lanthanum sulfide exhibits desirable thermoelectric properties over the temperature range of from about 1200.degree. C. to about 1500.degree. C. At temperatures below about 1200.degree. C. the crystal structure of the lanthanum sulfide changes from the cubic phase, which provides the desirable thermoelectric properties, to an orthorhombic form which lacks the desired thermo-electric properties. Such phase transformation can also produce a brittle, fragile material with insufficient strength to be used as a thermoelectric power conversion device.